Natural products from plants and microorganisms have proven to be a major source of active anticancer agents and lead compounds for cancer chemotherapy. Mushrooms of the class Basidiomycetes are an exception. Although they occur widely and some are well known to contain a variety of highly poisonous substances, only Omphalotus illudens (jack o""lantern mushroom) is known to produce promising anticancer compounds. These are the sesquiterpenes illudin S and illudin M. The illudins are extremely cytotoxic compounds but have a low therapeutic index particularly in solid tumor systems. However, modification of their structures has yielded several analogs, which possess a greatly improved therapeutic index. Remarkable efficacy has been observed in tests on mouse xenografts of leukemias and various solid tumors.
First and second generation analogs, for example, dehydroilludin M and acylfulvene, have been described (WO 91/04754). A promising compound is a third generation analog hydroxymethylacylfulvene (HMAF). In tests with MV 522 metastatic lung carcinoma xenografts in nude mice, complete tumor regression was observed in all animals. HMAF also exhibited outstanding activity against breast (MX-1), colon (HT-29) and skin cancers.
The structures of illudin S and illudin M were first published in 1963 (McMorris et al., J. Am. Chem. Soc. 85:831 (1963)). Until recently only one total synthesis of these compounds had been reported (Matsumoto et al., Tetrahedron Lett. 1171 (1970)). This synthesis involved Michael addition of a cycylopropane intermediate to an appropriately substituted cyclopentenone. The resulting product was then transformed into an intermediate which could undergo aldol condensation to form illudin""s six-membered ring. A number of further reactions were required to complete the synthesis.
Padwa et al., (J. Am. Chem. Soc. 116:2667 (1994)), have published a synthetic approach to the illudin skeleton using a dipolar cycloaddition reaction of a cyclic carbonyl ylide dipole with cyclopentenone to construct the six-membered ring. Kinder and Bair (J. Org. Chem. 59:6955 (1994)), have also employed the Padwa methodology to synthesize illudin M. However, these syntheses are long and not well suited for making acylfulvenes on a large scale.
Thus, a continuing need exists for improved methods for synthesizing acylfulvenes.
The present invention provides a method of synthesizing compounds of formula (I): 
wherein R and Rxe2x80x2 are independently (C1-C4)alkyl, preferably methyl. According to the invention, a method is provided of synthesizing a compound of formula (V), a preferred intermediate in the synthesis of compounds of formula (I),: 
comprising the steps of coupling a cyclopentanone of formula (II): 
wherein R4 is xe2x80x94Oxe2x80x94C(R9)2O(R9), wherein R9 is (C1-C4)alkyl, preferably methyl;
with a cyclic carbonyl ylide dipole of formula (III): 
to form a compound of formula (IV): 
and treating compound (IV) with base to form a ketone of formula (V).
The present method further may further comprise the steps of dihydroxylating the ketone to yield a compound of formula (VI): 
and treating the compound of formula (VI) with a removable 1,2-diol protecting reagent to yield an intermediate of formula (VII): 
wherein X is a removable 1,2-diol protecting group. Protecting groups may be introduced by forming a cyclic acetal by treatment with an aldehyde or ketone such as acetone, formaldehyde, acetaldehyde or benzaldehyde. For example, an isopropylidene derivative (acetonide) may be introduced by reaction with acetone. Preferably, the isopropylidene group is introduced by acid-catalyzed exchange with 2,2-dimethoxypropane.
The method further comprises the steps of treating compound (VII) with RMgCl, where R is (C1-C4)alkyl, to yield a Grignard product of formula VIII: 
and cleaving the oxybridge to yield a diol of formula (IX): 
The method further comprises the step of removing the diol protecting group to yield a tetraol of formula (X): 
The tetraol is then converted to an orthoester of formula (XI): 
wherein Rxe2x80x3 is (C1-C3)alkyl; and the cis hydroxyls are eliminated to yield a dienone of formula (XII): 
The method further comprises the steps of reducing the compound of formula (XII) to convert the ketone to an alcohol, under conditions which dehydrate the resulting alcohol to yield a fuilvene of formula (XIII): 
The fulvene of formula (XIII) is then oxidized to yield a compound of formula (I): 
The present invention also provides a method of synthesizing a compound of formula (XVII): 
wherein R1 is OH, R2 is H, and Rxe2x80x2 is (C1-C4)alkyl, preferably methyl.
According to the present invention, a method is provided of synthesizing a diketone of formula (XIII), a preferred intermediate in the synthesis of compounds of formula (XVII),: 
comprising the steps of
(a) cleaving the oxybridge in the compound of formula (XIV): 
to yield a diketone of formula (XIII).
The method further comprises the steps of
(b) protecting the hydroxyl group in the compound of formula (XIII) with a removable hydroxyl protecting group X; and
(c) introducing a double bond in the five-membered ring to yield a compound of the formula (XV): 
wherein Rxe2x80x21 and Rxe2x80x22 together are keto; and
X is a removable hydroxyl protecting group. Removable hydroxyl protecting groups may be introduced by reaction with a suitable reagent, such as a reagent of the formula ((C1-C4)alkyl)3SiCl, including triethylsilyl (TES) chloride, trimethylsilyl (TMS) chloride, t-butyldimethylsilyl (TBDMS) chloride, dimethyl (1,2,2-trimethylpropyl)silyl chloride, or tris(isopropyl)silyl; and methoxymethyl chloride, xcex2-methoxyethoxymethyl chloride, and isobutylene.
The method further comprises the steps of
(d) reducing both keto groups to yield hydroxy groups under conditions that yield a compound of formula (XVI): 
(e) eliminating the cyclopcntenol hydroxyl group; and
(f) oxidizing the cyclohexanol hydroxyl group and removing hydroxyl protecting group X to yield a compound of formula (XVII): 
wherein R1 is OH and R2 is H.
The method additionally comprises the step of
(g) following step (d), treating the alcohol with mesyl chloride in the presence of a base to produce a mesylate of the formula (XVIII): 
wherein Rxe2x80x31 is xe2x80x94OX, Rxe2x80x32 is absent and R is H.
The present invention further provides a method of synthesizing compounds of the formula (XXIII): 
wherein Rxe2x80x21 and Rxe2x80x22 together are ethylenedioxy, and Rxe2x80x2 is (C1-C4)alkyl, preferably methyl.
According to the present method, the carbonyl group of the compound of formula (XIII) is converted to an acetal group to yield a compound of formula (XIX): 
The method further comprises the steps of
(b) protecting the hydroxyl group in the compound of formula (XIX) with a removable hydroxyl protecting group X; and
(c) introducing a double bond in the five-membered ring to yield a compound of the formula (XX): 
wherein X is a removable hydroxyl protecting group.
The method further comprises the steps of
(d) reducing the keto group to yield a hydroxy group under conditions that yield a compound of formula (XXI): 
(e) eliminating the cyclopentenol hydroxyl group;
(f) removing hydroxyl protecting group X to yield a compound of formula (XXII): 
and
(g) oxidizing the cyclohexanol hydroxyl group to yield a compound of formula (XXIII): 
The method further comprises the step of
(h) following step (d), treating the alcohol with mesyl chloride to produce a mesylate of the formula (XXIV): 
With respect to both mesylates of formulas (XVIII) and (XXIV), the mesylates are relatively unstable and convert to fulvenes upon standing. Removal of the protecting group X and oxidation yield compounds of formulas (XVII) and (XXIII), respectively.
The invention also provides novel compounds of formula I-XXIV, all of which are useful as intermediates in the synthesis of 6-substituted acylfulvene analogs (6-substituted acylfulvenes) as disclosed, for example, in Kelner et al., U.S. Pat. No. 5,523490, or which have antitumor or cytotoxic activity per se.